Sissy Bar Different Version

[Yogibear]

I'm in the process of creating a different sissy bar for my Rocket rather than the Triumph one and wondered if anyone could help me with dimensions.

I've got some plates from Mittzy of R3owners and also a second hand triumph genuine rack but want to have a round sissy bar rather than the Triumph square tube similar to the Harley one below



So my question is what is the distance between the standard triumph plates where the uprights would fit and also has anyone done anything like this?

Thanks in Advance